1. Field of the Invention
The present invention relates to a nonaqueous electrolyte secondary battery.
2. Description of the Related Art
The development of a nonaqueous electrolyte secondary battery, in which the charge-discharge is carried out by the migration of the Li ions between the negative electrode and the positive electrode, is being carried out vigorously as a battery of a high energy density. It is desirable for the negative electrode active material used in the nonaqueous electrolyte secondary battery to have a large capacity and a long cycle life.
A carbonaceous material is used nowadays as the negative electrode active material. The carbonaceous material has a long cycle life because the Li absorption-release reaction is carried out by the reversible intercalation reaction. However, the carbonaceous material is small in its Li absorption amount so as to give rise to the problem that the capacity of the carbonaceous material is small.
Such being the situation, metals and alloys, which are expected to exhibit a large capacity, are being studied as the negative electrode active material.
The metals such as Al, Si, Ge, Sn and Sb, which exhibit a high reactivity with Li, were studied as the negative electrode active material. However, in the case of these metals, the Li absorbing-releasing reaction is carried out by the alloying reaction, leading to a short cycle life. It should be noted in this connection that the negative electrode active material is repeatedly subjected to the change in crystal structure and to the expansion-shrinkage in accordance with progress of the alloying reaction. As a result, the negative electrode active material is finely pulverized so as to decrease the amount of Li contributing to the charge-discharge of the secondary battery. Such being the situation, the cycle life of the negative electrode active material is shortened as pointed out above.
The alloy materials having a specified composition including a rare earth element and Sn, which are manufactured by the process including the melting step, the cooling step and the heat treating step that are carried out under specified conditions, are proposed as the alloys effective for forming the negative electrode active material in Japanese Patent Disclosure (Kokai) No. 2003-197188. This patent document refers to, for example, CeSn3 as a binary alloy and to La0.6Co0.4Sn3 as a ternary alloy.
However, the Li absorbing-releasing reaction carried out by the alloys disclosed in the patent document referred to above is provided by the alloying reaction. As a result, it was impossible for the negative electrode active material formed of the particular alloys to exhibit a long cycle life.